The present invention relates to flash photography using an electronic flash device, and more particularly to a flash photography system having a camera equipped with a focal plane shutter.
When an object illuminated by a flash is to be photographed by a camera equipped with a focal plane shutter, exposure of a photograph is uneven along the direction of movement of a shutter closing curtain unless the light emission of the flash device started by the movement of a shutter opening curtain when an exposing aperture is fully opened is terminated before the shutter closing curtain appears in the aperture. Accordingly, in flash photography, it is necessary to set a shutter time or an exposure time long enough to avoid unevenness of exposure.
It has been known to detect the power-on of the flash device or the termination of charging of a main capacitor and supply a detected signal to a camera in order to automatically set the shutter time to a time in which no unevenness of exposure will occur. For example, see U.S. Pat. No. 4,016,575. Even in a compact flash device having a relatively short light emission duration, the shutter time is automatically set to the time corresponding to that of a large flash device having a relatively long light emission duration so that it can also be applicable to the large flash device. As a result, even if the flash device having the short light emission duration is used, a stop motion effect cannot be fully utilized.
Japanese Laid-Open Patent Application No. 91217/1974 (laid open on Aug. 31, 1974) discloses a technique to prevent the unevenness of exposure by forcibly stopping the light emission of the flash device by an electric signal which triggers the start of movement of the shutter closing curtain. In this method, however, a full quantity of light emitted by the flash device cannot be utilized and an underexposure condition due to the interruption of the light emission cannot be predicted before photographing. Thus, when the full quantity of light is not utilized, a range of distance which allows automatic light emission control in accordance with distance to an object is reduced by an unpredictable amount.
It has also been proposed to supply an electric signal which stops the light emission of an automatic light emission control type electronic flash device to a camera to start the closing operation of the shutter closing curtain. However, since it takes at least 2-3 milliseconds before the shutter closing curtain actually start to close the exposing aperture, a very long shutter time is needed and hence the photographing may be subject to the influence of a light source other than the flash device and an unclear or unnaturally colored photograph may be produced.